The Bosch process is a plasma etch process that has been widely used to fabricate deep vertical (high aspect ratio) features (with depth such as tens to hundreds of micrometers), such as trenches and vias, in the semiconductor industry. The Bosch process comprises cycles of alternating etching steps and deposition steps. Details of the Bosch process can be found in U.S. Pat. No. 5,501,893, which is hereby incorporated by reference. The Bosch process can be carried out in a plasma processing apparatus configured with a high-density plasma source, such as an inductively coupled plasma (ICP) source, in conjunction with a radio frequency (RF) biased substrate electrode. Process gases used in the Bosch process for etching silicon can be sulfur hexafluoride (SF6) in an etching step and octofluorocyclobutane (C4F8) in a deposition step. The process gas used in the etching step and the process gas used in the deposition step are respectively referred to as “etch gas” and “deposition gas” hereinbelow. During an etching step, SF6 facilitates spontaneous and isotropic etching of silicon (Si); during a deposition step, C4F8 facilitates the deposition of a protective polymer layer onto sidewalls as well as bottoms of the etched structures. The Bosch process cyclically alternates between etch and deposition steps enabling deep structures to be defined into a masked silicon substrate. Upon energetic and directional ion bombardment, which is present in the etching steps, any polymer film coated in the bottoms of etched structures from the previous deposition step will be removed to expose the silicon surface for further etching. The polymer film on the sidewall will remain because it is not subjected to direct ion bombardment, thereby, inhibiting lateral etching.
U.S. Patent Publication No. 2009/0242512 discloses an example of a multi-step Bosch type process in which the chamber pressure is at 35 mTorr for 5 seconds during deposition of a passivation film, 20 mTorr for 1.5 seconds during a low pressure etch step and 325 mTorr for 7.5 seconds during a high pressure etch step (see Table 4.2.1) or 35 mTorr for 5 seconds during deposition, 20 mTorr for 1.5 seconds during low pressure etch, 325 mTorr for 7.5 seconds during high pressure etch and 15 mTorr for 1 second during low pressure etch (see Table 4.2.2).
Variation in chamber pressure is desired in other processes such as atomic layer deposition, plasma enhanced CVD, multi-step processes of plasma etching openings in mask material and removal of the mask material, multi-step plasma etch processes wherein the concentration of etchant gas is periodically varied or different layers of material are sequentially etched. To reduce the overall processing time, reduction in the transition period between high and low pressure phases of such cyclical processes would be desirable. For instance, U.S. Patent Publication No. 2009/0325386 discloses a conductance limiting element for rapid adjustment of pressure in a low volume vacuum chamber on the order of tens of milliseconds. The '386 publication states that during processing, a single chemical species can be flowed in the processing region during multiple pressure cycles or different chemical species can be introduced during multiple pressure cycles with the time at high or low pressure ranging from 0.1 to 2 seconds.